La Plagne, France ? Lance Armstrong is dwelling on a bitter defeat, even as he closes in on his fourth straight victory in the Tour de France.

Armstrong extended his overall lead to more than five minutes Wednesday, finishing third behind Dutch rider Michael Boogerd in the grueling 16th stage – the most difficult of the three-week Tour.

The 30-year-old Texan finished well ahead of his nearest rival, Spain’s Joseba Beloki, and stretched his lead from 4 minutes, 21 seconds to 5:06 – a huge margin now that the Tour’s toughest stages are over.

Barring illness or injury, Armstrong is almost certain to win a fourth straight title when the Tour ends on Sunday.

Yet, with one mountain stage remaining on Thursday, Armstrong couldn’t relax.

“It’s a complicated day,” Armstrong said of the run from Aime to Cluses in the Alps. “It’s never flat. It’s the same stage as two years ago when I cracked at Joux-Plane.”

Thursday’s 88-mile stage is not identical to the stage from Courchevel to Morzine where Armstrong experienced his only weak moment of the 2000 Tour.

Armstrong suddenly tired while pushing up the Col de Joux-Plane pass in the final mountain stage two years ago. The U.S. Postal Service team rider finished the leg far behind his archrival, Jan Ullrich of Germany, and lost nearly two minutes off his overall lead.

Armstrong blamed lack of food, and sugar, especially, for his loss of form.

That was his last weak spell in any Tour de France.

In 2001, he was flawless throughout, and this year he has consistently increased his lead over Beloki.

Armstrong’s lead in this Tour is less than the 6:15 advantage he held over Fernando Escartin after 16 stages in 1999. It is also less than the 5:37 lead he had over Ullrich in 2000, after a disastrous day at Joux-Plane.

His lead is a second more that the advantage he had over Ullrich in 2001.

Thursday’s stage features four tough climbs, but none that compare to the three climbs in Wednesday’s leg from Les Deux-Alpes to La Plagne. Also, he won’t have to face Joux-Plane.

Armstrong is also expected to extend his lead in Saturday’s individual time trial.

He finished last year’s race with a 6:44 advantage over Ullrich, aided, in part, by his stunning time-trial run.

Boogerd, of the Rabobank team, won Wednesday’s 111.29-mile stage in 5:48.29.

Spain’s Carlos Sastre was second, just ahead of Armstrong. Both finished 1:25 behind Boogerd, and 37 seconds ahead of Beloki. Armstrong earned 8 bonus seconds for finishing third.

Boogerd said he hadn’t thought about being overtaken by Armstrong in the final stretch.

“I was only suffering and fighting to make the stage,” he said.

Boogerd, who also won a stage in the 1996 Tour, called Wednesday’s victory “probably the most beautiful win of my career.”

Armstrong was congratulated by Prince Albert of Monaco after the stage.